An industrial engineering approach was used to determine what proportion of a rail vehicle fleet would need to be unavailable for revenue use. This proportion is often called the spare ratio (SR). The number of rail vehicles required as spares remains in dispute. This SR question is important because each spare rail car and locomotive represents an investment of millions of dollars. The prevalent approach for determining the number of spares is to use a fixed ratio, for example, 15%. This fixed percentage is not a function of several variables, or even property-specific parameters. The industrial engineering approach shows that an SR is a variable that can change annually. The significant variables driving the SR are annual use, reliability, inventory support, train length, and repair scheduling aggressiveness. Equations were developed to enable calculations specific to a transit or commuter rail operation. Further, the duration of the peak vehicle requirement (PVR)--peak hours--was shown to be as important as the PVR size.